1. Field of the Invention
The present invention relates to a display device and a driving method thereof. In particular, the present invention relates to a display device to which an area gray scale method is applied and a driving method thereof.
2. Description of the Related Art
In recent years, a so-called self-luminous type display device having a pixel that is formed of a light emitting element such as a light emitting diode (LED) has been attracting attention. As a light emitting element used for such a self-luminous type display device, an organic light emitting diode (OLED) (also called an organic EL element, an electro luminescence: EL element, and the like) has been drawing attention and used for an EL display. Since a light emitting element such as an OLED is a self-luminous type, it has advantages such as higher visibility of pixels than that of a liquid crystal display, and fast response without requiring a backlight. The luminance of a light emitting element is controlled by a current value flowing through the light emitting element.
As a driving method for controlling a light emitting gray scale of such a display device, there are a digital gray scale method and an analog gray scale method. In accordance with the digital gray scale method, a light emitting element is turned on/off in a digital manner to express a gray scale. Meanwhile, the analog gray scale method includes a method for controlling the light emission intensity of a light emitting element in an analog manner and a method for controlling the light emitting period of a light emitting element in an analog manner.
In the case of the digital gray scale method, there are only two states: a light emitting state and a non-light emitting state. Therefore, only two gray scales can be expressed if nothing is done. Accordingly, another method is used in combination to achieve multiple gray scales. An area gray scale method and a time gray scale method are often used as a method for multiple gray scales.
The area gray scale method is a method for expressing a gray scale by controlling an area of a lighting portion. In other words, gray scale display is performed by dividing one pixel into a plurality of sub-pixels and controlling the number or area of lighted sub-pixels (for example, see Reference 1: Japanese Patent Application Laid-Open No. H11-73158 and Reference 2: Japanese Patent Application Laid-Open No. 2001-125526). In the area gray scale method, the number of the sub-pixels cannot be increased; therefore, it is difficult to realize high-definition and multiple gray scales. This can be given as a disadvantage of the area gray scale method.
The time gray scale method is a method for expressing a gray scale by controlling the length of a light emitting period or the frequency of light emission. In other words, one frame is divided into a plurality of sub-frames, each of which is weighted with respect to the number of light emissions and a light emitting period, and then the total weight (the sum of the frequency of light emission and the sum of the light emitting period) is differentiated for each gray scale, thereby expressing a gray scale. It is known that display failure such as a pseudo contour (or a false contour) may occur when such a time gray scale method is used and measures against the failure is considered (for example, see Reference 3: Patent Publication No. 2903984, Reference 4: Patent Publication No. 3075335, Reference 5: Patent Publication No. 2639311, Reference 6: Patent Publication No. 3322809, Reference 7: Japanese Patent Application Laid-Open No. H10-307561, Reference 8: Patent Publication No. 3585369, and Reference 9: Patent Publication No. 3486884).
However, even though various methods for reducing pseudo contour have been suggested, an effect of sufficiently reducing pseudo contour has not been obtained.
For example, FIG. 1 in Reference 4 is referred to, and it is assumed that a gray scale of 127 is expressed in a pixel A and a gray scale of 128 is expressed in a pixel B adjacent to the pixel A. A light emitting state and a non-light emitting state in each sub-frame of this case are shown in FIGS. 60A and 60B. For example, FIG. 60A shows a case of seeing only the pixel A or the pixel B without turning one's eyes away. A pseudo contour is not caused in this case. This is because one's eyes sense the brightness according to the sum of the brightness of the place where one's line of sight passes. Thus, eyes sense the gray scale of the pixel A to be 127 (=1+2+4+8+16+32+32+32), and eyes sense the gray scale of the pixel B to be 128 (=32+32+32+32). In other words, eyes sense an accurate gray scale.
On the other hand, FIG. 60B shows a case where a line of sight is moved from the pixel A to the pixel B or from the pixel B to the pixel A. In this case, a gray scale is at times perceived to be 96 (=32+32+32), and at other times the gray scale is perceived to be 159 (=1+2+4+8+16+32+32+32+32) depending on a movement of the line of sight. Even though the gray scale is supposed to be perceived as 127 and 128, the gray scale is perceived to be 96 or 159, and pseudo contour occurs.
FIGS. 60A and 60B show a case of 8-bit gray scale (256 gray scales). Next, FIG. 61 shows a case of 6-bit gray scale (64 gray scales). In this case also, eyes sometimes sense the gray scale to be 16 (=16), and sometimes sense the gray scale to be 47 (=1+2+4+8+16+16) in accordance with eyes' movement. Although the eyes are supposed to sense the gray scales to be 31 and 32, they sense the gray scales to be 16 or 47. Consequently, a pseudo contour is caused.